1. Field of the Invention
The invention relates to display apparatus and, more particularly, to a display apparatus using a display panel having a memory performance such as a ferroelectric liquid crystal display panel.
2. Related Background Art
In a CRT (cathode ray tube) which forms an image by using a decay characteristic of a fluorescent material or a TN (twisted nematic) type LCD (liquid crystal device) which forms an image by using a transmission light amount characteristic according to an effective value of a driving voltage, it is necessary to keep a frame frequency as one picture plane forming frequency to predetermined value or more from a viewpoint of the display principle. Generally, it is held to 30 Hz or higher. The frame frequency can be expressed by an inverse number of the product of the number of scanning lines constructing a display section and a horizontal scan time to scan the scanning lines. In the present situation, an interlacing method (jumping scan of every other scanning lines) and a non-interlacing method (non-jumping scan) have been known as scanning methods. A pairing method, a simultaneous parallel scanning method whereby the screen is divided into a plurality of display areas and the areas are simultaneously scanned in parallel although such a method is limited to the LCD, and the like have been put into practical use as another methods. In the NTSC standard, there is used the interlacing method of two fields/frame having a frame frequency of 30 Hz, in which the horizontal scan time is set to about 63.5 .mu. sec and the number of scanning lines is set to about 480 (the number of effective display lines). In the case of the TN type LCD, there is used the non-interlacing method in which the number of scanning lines is set to a value within a range from 200 to 400 and the frame frequency is set to 30 Hz or higher. In the CRT, separately from the NTSC standard, the non-interlacing method of a frame frequency of about 40 to 60 Hz is also used and the number of scanning lines is set to a value within a range from about 200 to 1000.
The cases of driving the CRT and the TN type LCD each of which is constructed by 1920 pixels in the vertical direction (scanning lines) .times.2560 pixels in the lateral direction (data lines) will now be considered. In the case of using the interlacing method of a frame frequency of 30 Hz, the horizontal scan time is equal to about 17.5 .mu.sec and the horizontal dot clock frequency is equal to about 147 MHz (no consideration is made with respect to the horizontal blanking time in the CRT). In the case of the CRT, the horizontal dot clock frequency of 147 MHz needs a very high beam scanning speed and fairly exceeds the maximum electron beam modulating frequency of the electron gun in the present image receiving tube. Even if the electron beam is scanned at a speed of 17.5 .mu.sec, a video image cannot be accurately displayed. In the case of the TN type LCD, the driving of 1920 scanning lines corresponds to a duty ratio of 1920. Such a duty ratio is fairly larger than the present maximum duty ratio of about 400, so that an image cannot be displayed. Therefore, when considering the case of driving by setting the horizontal scan time as an actual value, the frame frequency is smaller than 30 Hz, so that the scanning state is visually recognized or a flickering occurs and the display quality is remarkably deteriorated. As mentioned above, it is a present situation that there are limitations in the realization of a large screen and a high density of the CRT and the TN type LCD because the number of scanning lines cannot be increased due to the display principle and the limitation of the drive elements or the like.
In recent years, Clerk and Lagerwell have proposed a ferroelectric liquid crystal device having a high response speed and a memory performance (bistability) by U.S. Pat. Ser. No. 4,367,924 or the like.
The ferroelectric liquid crystal device generally has a Chiral smectic C phase (SmC*) or H phase (SmH*) in a special temperature range. In this state, the ferroelectric temperature range. In this state, the ferroelectric liquid crystal device is set into either one of the first and second optical stable states in response to an electric field which is applied and has a characteristic, namely, a bistability such that the state is maintained when no electric field is applied. In addition, a response speed for a change in electric field is also high. Therefore, a wide use of such a device is expected as a high speed display device of the memory type.
However, generally, it is difficult that the ferroelectric liquid crystal device has the bistability as proposed by Clerk et al. and there is a large tendency such that the device has a monostable state. To realize the permanent bistability, Clerk et al. have used an orientation control method by applying a shearing force by sharing or by applying a magnetic field or the like. However, a method whereby a uniaxial orientation process such as rubbing process, oblique evaporation depositing process, or the like is executed to a substrate is advantageous as an orientation control method from a viewpoint of the production technique. There is a case where a permanent bistability is not obtained in the ferroelectric liquid crystal device whose orientation has been controlled by executing such a uniaxial orientation process to the substrate. The orientation state such that the permanent bistability doesn't occur, that is, what is called a monostable orientation state has a nature such that a biaxial orientation when the electric field has been applied is changed to the uniaxial orientation within a range from a few msec to several hours when no electric field is applied. Therefore, the display apparatus using the monostable ferroelectric liquid crystal device has a problem such that the image which has once been written is extinguished by cancelling the supply of the electric field. Particularly, upon multiplexing driving, there is a problem such that the writing states of the pixels on the scanning lines which are not accessed are gradually extinguished.
Therefore, to solve the above problem, there is considered a driving method (refreshing drive) whereby a voltage signal to cause "black" in the pixels on the selected scanning line and a voltage signal to cause "white" are selectively applied and, when it is assumed that a period to sequentially select the scanning lines is set to one frame or a plurality of fields, by repeating such a period, the writing process is executed. By using such a refreshing driving method, a fluctuation of the transmission light amount of the non-selected pixel is very small. Moreover, even at a frame frequency lower than 30 Hz, the visual recognition of the writing scanning line (phenomenon such that the scan writing line has a luminance higher than those of the other lines and can be also visually easily discriminated) and the occurrence of the flickering can be eliminated. In this case, by the examinations by the inventors of the present invention, it could be confirmed that a similar effect can be obtained even at a frame frequency of about 5 Hz.
The above fact is effective to solve in a lump the problems in the realization of a large screen and a high precision which occur from the inevitable condition such that the device must be driven at a frame frequency of 30 Hz or higher as a limit frequency in the CRT and then TN type LCD mentioned above.
However, in the case of refresh-driving at such a low frame frequency as mentioned above, there is a problem such that a processing speed at such a low frequency is slow for what is called a moving image display such as smooth scroll, cursor movement, or the like upon character edition or on a graphic screen, or the like and the display performance is deteriorated. In recent years, the developments of computers, peripheral circuits, and a software are remarkable. Particularly, for the display of a large screen and a high precision, a display method called a multi-window in which a plurality of screens are overlappingly displayed in the display area has widely been used. The display apparatus using the ferroelectric liquid crystal device can realize a large screen and a high precision which are extremely superior to those of the conventional display apparatuses (CRT, TN type LCD, and the like). However, there is a problem such that the frame frequency becomes low in association with the realization of the large screen and high precision, so that the speeds of the smooth scroll and the cursor movement become more and more slow.